simulator/heater_sim.c: Add notes that the heater isn't implemented in the simulator.
heater.*:Adjust DEFINE_HEATER macro to provide PID values.
config.issue74.h: M130-M136 set and report the PID values.
heater.c: Add units comments and sum the PID in consistent units, changing internal kP,kI,kD meanings.
config.issue74.h: Add 'MCU_TARGET=atmega644p  make -f Makefile-AVR' command line note.
heater.*, config.issue74.h: Switch from Ti,Td to kI, Kd.  Add notes, and resolve TH_COUNT scaling of kD.